1. Field of the Invention
The present invention relates to a lithium manganese oxide having a spinel structure, and a process for its production and a lithium secondary cell employing it.
Manganese oxide is a material which has been used from long ago as a cell active material, and a lithium manganese composite oxide which is a composite material of manganese and lithium, is a material which has attracted an attention in recent years as a positive electrode active material for a lithium secondary cell.
2. Discussion of Background
A lithium secondary cell has attracted an attention as a new high performance cell along with miniaturization and light weight of electronic devices in recent years, because of its high energy density and high output.
The positive electrode material for a lithium secondary cell is required to have a high voltage-operational range, a high discharge capacity and a high cycle stability, and composite oxides of Li with various metals such as Co, Ni, Mn, V, etc. are being studied. LiMn2O4 having a spinel structure, which is one of composite oxides of Li and Mn, is considered to be prospective as a positive electrode active material, but it has a problem that it can hardly be reversibly cycled over a long period of time, and the electrochemical capacity of such a cell gradually decreases. Especially when it is operated under a high temperature condition of from 50 to 60xc2x0 C., the reduction of the electrochemical capacity has been found remarkable.
Further, since it is a composite oxide of Li and Mn, there will be a problem that the composition tends to fluctuate when a lithium manganese oxide having a spinel structure is prepared, which in turn adversely affect the cell performance when the lithium manganese oxide having a spinel structure is used as a positive electrode for a lithium secondary cell. Various proposals have been made for a production method to increase the uniformity such as a liquid phase synthesis. However, their practical application has been difficult for such reasons that the starting material are expensive, the reaction is vigorous, and the apparatus is expensive.
It is an object of the present invention is to provide a high performance lithium manganese oxide having a spinel structure excellent in cycle stability for a long period of time as a positive material for a Li secondary cell, and a process for its production whereby a lithium manganese oxide having a spinel structure, having a high uniformity, can be obtained by a solid phase reaction employing inexpensive starting materials, and a high performance lithium secondary cell employing it.
As a result of an extensive study by the present inventors, it has been found that the above object can be accomplished by a lithium manganese oxide having a spinel structure (Fd{overscore (3)}m), which is represented by general formula {Li}[LixMn2xe2x88x92x]O4, wherein { } represents 8a site, [ ] represents 16d site, and 0.00 less than xxe2x89xa60.50, and which has a Na content of at most 0.1 wt % and an average primary particle size of from 0.5 to 2.0 xcexcm as observed by SEM.
Further, it has been found that the above object can be accomplished by a process for producing a lithium manganese oxide having a spinel structure, which comprises mixing a lithium material and a manganese material, followed by calcination, wherein the ratio of the average particle size of the lithium material to that of the manganese material is from 1/5 to 1/30, as a process for producing a lithium manganese oxide having a spinel structure of the present invention.
As another process, it has been found that the above object can be accomplished by a process for producing a lithium manganese oxide having a spinel structure, which comprises mixing a lithium material and a manganese material, followed by calcination, wherein mixing of the lithium material and the manganese material is carried out while cooling.
As still another process, it has been found that the above object can be accomplished by a process for producing a lithium manganese oxide having a spinel structure, which comprises.mixing a lithium material and a manganese material, followed by calcination, wherein provisional heat treatment at a temperature of lower than 900xc2x0 C., main heat treatment at a temperature of from 750 to 950xc2x0 C., and further post heat treatment at a temperature of from 600 to 900xc2x0 C., are carried out.
Further, a high performance lithium secondary cell employing the lithium manganese oxide having a spinel structure of the present invention as a positive electrode active material, has been found. The present invention has been accomplished on the basis of these discoveries.